ABSTRACT
Structural studies of the Ag(I)-bound mbd4 fragment of Menkes’ Cu-transporting ATPase have been undertaken using NMR techniques [24]. Ag(I) was used due to the low stability of Cu(I) in solution, relative to Cu(II), and because it is predicted to have a similar structural environment. Two cysteine residues, C14 and C l7, near the top of the fragment (see Fig. 8) were identified as binding ligands. Their sulfur atoms are some 4.8(2) A apart and could therefore readily accommodate an S-M-S linkage where M is Ag(I) or Cu(I). T13 and S16, residues that appear to be conserved within the Menkes’ metal-binding domains flank this binding site, and the hydroxyl groups of these side chains may also interact with the metal cation. However, there are at present insufficient experimental data to confirm the / i dihedral angles for these residues. Residues 121 and F66, which are also conserved, may be important for stabilizing the metal-binding motif by packing against C l7 and L38 packs beneath the metal-binding loop in contact with C14. Clearly such a binding site, with the metal held by two cysteines on a surface exposed loop, would make the metal cation readily accessible for participation in the type of exchange mechanism that has been proposed for other copper chaperones [59].
